It is unclear what combination of CPU features is safe.

Location

https://doc.rust-lang.org/rustc/targets/known-issues.html

Summary

The linked page says:

Most target-feature problems arise, when mixing code that have the target-feature enabled with code that have it disabled. If you want to avoid undefined behavior, it is recommended to build all code (including the standard library and imported crates) with a common set of target-features.

By default, compiling your code with the -C target-feature flag will not recompile the entire standard library and/or imported crates with matching target features. Therefore, target features are generally considered as unsafe. Using #[target_feature] on individual functions makes the function unsafe.

Since this guide only provides examples of what's not okay (and calls them out explicitly as incomplete), and doesn't provide any guarantees about what is okay, a strict reading suggests that it's basically never okay to use target features at all (unless you are an expert on all the possible variations of your platform's ABI). A normal user who is not an expert has no way of knowing that any given feature is okay to enable so if they see these docs, they will have to be conservative and enable nothing.

I think this is an oversight, because I can't imagine that e.g. it's unacceptable/UB to enable mainstream SIMD features like AVX .

Start with the target-feature section in rustc/targets/known-issues.html and review the quoted guidance about mixing enabled and disabled code. The work is complete when the page explains which CPU-feature combinations are safe or provides clear guarantees beyond the current examples of unsafe usage.